Current practice for alcohol measurements is based upon either blood measurements or breath testing. Blood measurements are generally considered the most accurate for determining alcohol intoxication levels. However, blood measurements require a venous or capillary sample and involve significant handling precautions in order to minimize health risks. Once extracted, the blood sample must be properly labeled and transported to a clinical laboratory or other suitable location. A clinical gas chromatograph is typically used to measure the alcohol level in the sample. Due to the invasiveness of the procedure and the amount of sample handling involved, blood alcohol measurements are usually limited to critical situations such as for traffic accidents, violations where the suspect requests this type of test, and accidents where injuries are involved.
Because it is less invasive, breath testing is more commonly encountered in the field. In breath testing, the subject must expire air into the instrument for a sufficient time and volume to achieve a stable breath flow that originates from the alveoli deep within the lungs. The device then measures the alcohol content in the air, which is related to blood alcohol through a breath-blood partition coefficient. The blood-breath partition coefficient used in the United States is 2100 (implied units of mg EtOH/dL blood per mg EtOH/dL air) and varies between 1900 and 2400 in other nations. The partition coefficient is highly subject dependent. In other words, each subject will have a partition coefficient in the 1900 to 2400 range, depending on his or her physiology. Since knowledge of each subject's partition coefficient is unavailable in field applications, each nation assumes a single partition coefficient value that is globally applied to all measurements. In the U.S., defendants in DUI cases often use the globally applied partition coefficient as an argument to impede prosecution.
Breath measurements have additional limitations. First, the presence of “mouth alcohol” can falsely elevate the breath alcohol measurement. This necessitates a 15-minute waiting period prior to making a measurement in order to ensure that no mouth alcohol is present. For a similar reason, a 15 minute delay is required for individuals who are observed to burp or vomit. A delay of 10 minutes or more can be required between breath measurements to allow the instrument to return to equilibrium with the ambient air and zero alcohol levels. In addition, the accuracy of breath alcohol measurements is sensitive to numerous physiological and environmental factors.
Multiple government agencies, and society in general, seek non-invasive alternatives to blood and breath alcohol measurements. Quantitative spectroscopy offers the potential for a completely non-invasive alcohol measurement that is not sensitive to the limitations of the current measurement methodologies. While non-invasive determination of biological attributes by quantitative spectroscopy has been found to be highly desirable, it has been very difficult to accomplish. Attributes of interest include, as examples, analyte presence, analyte concentration (e.g., alcohol concentration), direction of change of an analyte concentration, rate of change of an analyte concentration, disease presence (e.g., alcoholism), disease state, and combinations and subsets thereof. Non-invasive measurements via quantitative spectroscopy are desirable because they are painless, do not require a fluid draw from the body, carry little risk of contamination or infection, do not generate any hazardous waste, and can have short measurement times.
Several systems have been proposed for the non-invasive determination of attributes of biological tissue. These systems have included technologies incorporating polarimetry, mid-infrared spectroscopy, Raman spectroscopy, Kromoscopy, fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, radio-frequency spectroscopy, ultrasound, transdermal measurements, photo-acoustic spectroscopy, and near-infrared spectroscopy. However, these systems have not replaced direct and invasive alcohol measurements.
As an example, Robinson et al. in U.S. Pat. No. 4,975,581 disclose a method and apparatus for measuring a characteristic of unknown value in a biological sample using infrared spectroscopy in conjunction with a multivariate model that is empirically derived from a set of spectra of biological samples of known characteristic values. The above-mentioned characteristic is generally the concentration of an analyte, such as alcohol, but also can be any chemical or physical property of the sample. The method of Robinson et al. involves a two-step process that includes both calibration and prediction steps.
In the calibration step, the infrared light is coupled to calibration samples of known characteristic values so that there is attenuation of at least several wavelengths of the infrared radiation as a function of the various components and analytes comprising the sample with known characteristic value. The infrared light is coupled to the sample by passing the light through the sample or by reflecting the light off the sample. Absorption of the infrared light by the sample causes intensity variations of the light that are a function of the wavelength of the light. The resulting intensity variations at a minimum of several wavelengths are measured for the set of calibration samples of known characteristic values. Original or transformed intensity variations are then empirically related to the known characteristics of the calibration samples using multivariate algorithms to obtain a multivariate calibration model. The model preferably accounts for subject variability, instrument variability and environment variability.
In the prediction step, the infrared light is coupled to a sample of unknown characteristic value, and a multivariate calibration model is applied to the original or transformed intensity variations of the appropriate wavelengths of light measured from this unknown sample. The result of the prediction step is the estimated value of the characteristic of the unknown sample. The disclosure of Robinson et al., U.S. Pat. No. 4,975,581, is incorporated herein by reference.
A further method of building a calibration model and using such model for prediction of analytes and/or attributes of tissue is disclosed in commonly assigned U.S. Pat. No. 6,157,041 to Thomas et al., entitled “Method and Apparatus for Tailoring Spectrographic Calibration Models,” the disclosure of which is incorporated herein by reference.
In U.S. Pat. No. 5,830,132, Robinson describes a general method of robust sampling of tissue for non-invasive analyte measurement. The sampling method utilizes a tissue-sampling accessory that is pathlength optimized by spectral region for measuring an analyte such as alcohol. The patent discloses several types of spectrometers for measuring the spectrum of the tissue from 400 to 2500 nm, including acousto-optical tunable filters, discrete wavelength spectrometers, filters, grating spectrometers and FTIR spectrometers. The disclosure of Robinson, U.S. Pat. No. 5,830,132, is incorporated herein by reference.
In “New Approach to High-Precision Fourier Transform Spectrometer Design”, Applied Optics, 35:16, 2891-2895, 1996, Brault introduces a constant time sampling analog-to-digital conversion technique for FTIR spectrometers that allows use of high dynamic range delta-sigma ADCs. Brault asserts their approach provides a superior technique for implementing the data acquisition system of an FTIR spectrometer because it avoids the artifacts of gain ranging and the need to precisely match the time delays between the laser reference and infrared measurement channels. In “Uniform Time-Sampling Fourier Transform Spectroscopy”, Applied Optics, 36:1-, 2206-2210, 1997, Brasunas et al. discuss a variation of Brault's constant time sampling analog-to-digital conversion technique for FTIR spectrometers.
In U.S. Pat. No. 5,914,780, Turner et al. describe a method of digitizing the interferogram of an FTIR spectrometer using a constant time sampling analog-to-digital converter. The constant time sampling technique allows the use of high dynamic range, delta-sigma analog-to-digital converters that obviate the need for gain ranging circuitry and precisely matched delays between the reference laser and infrared signals. This type of data acquisition system is asserted to provide the FTIR spectrometer with higher SNR and superior photometric accuracy when compared to the previously employed sampling technique which is triggered by the zero crossings of the reference laser.
Although there has been substantial work conducted in attempting to produce commercially viable non-invasive near-infrared spectroscopy-based systems for determination of biological attributes, no such device is presently available. It is believed that prior art systems discussed above have failed for one or more reasons to fully meet the challenges imposed by the spectral characteristics of tissue which make the design of a non-invasive measurement system a formidable task. Thus, there is a substantial need for a commercially viable device that can provide sufficient accuracy and precision.